Protecting a basement from flooding: backwater valves, sump pumps, downspouts and grading

A finished basement turns a flood from a mop-up into an insurance claim. The cheapest protection is installed before the finishing starts, and the federal Get Prepared program lists what matters most.

Backwater valves

A backwater valve on the sewer line closes when sewage starts flowing back into the house. The guidance recommends installation by a licensed plumber, cleaning at least twice a year, and a valve with an alarm so you do not flood yourself by using water while it is closed. Many municipalities require them in new homes; some offer subsidies for existing homes.

Sump pumps

Sump pumps move water away from the foundation where drainage cannot keep up. The guidance says every sump pump should have an alternate power source and a backup pump, be fully tested once a year, and be replaced about every 10 years.

Eavestroughs and downspouts

Downspouts should discharge to a well-drained area at least 2 m (6 ft) from the foundations of all nearby homes, if possible. Connecting downspouts to the sewer can overwhelm the storm system in heavy rain.

Grading

The ground should slope down and away from the house. The guidance cites about 2% as the minimum slope often recommended, and about 5% as ideal next to the foundation. Check grading every spring and fall.

Inside the basement

Seal basement windows and ground-level doors, raise appliances off the floor, and keep floor drains clear.

Prevention Checklist

• Have a licensed plumber install a backwater valve, preferably with an alarm.
• Fit the sump pump with backup power and a backup pump.
• Extend downspouts at least 2 m from foundations.
• Regrade so the ground slopes away from the house.
• Check your municipality for required devices and subsidies.

References

1. Government of Canada (Get Prepared). Backwater valves, Web page, Installation, cleaning, alarm, municipal requirements. Accessed 2026-11-01.
2. Government of Canada (Get Prepared). Sump pumps, Web page, Backup power, testing, replacement. Accessed 2026-11-01.
3. Government of Canada (Get Prepared). Eavestroughs and downspouts, Web page, 2 m discharge; sewer connections. Accessed 2026-11-01.
4. Government of Canada (Get Prepared). Property grading, Web page, 2% and 5% slopes. Accessed 2026-11-01.

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